Fantasy Aquarium by DS is a fish tank simulation. Contrary to Aquarium by DS by the same developer, which is limited to "normal" aquarium suiting fishes, the player of this game can choose from 30 different underwater animals which can't be held in fish tanks in real life (e.g. whales, dolphins, sharks, penguins). Additionally, the player can customize their tank in size and add different backgrounds and accessories like plants, rocks or even buildings like the Leaning Tower of Pisa. New fish types can be unlocked by taking good care of the aquariums residents.
And that's the aim of the game, taking care of the fishes in the tank. They need to be fed, the tank has to be cleaned and water temp and salinity have to be adjusted to create a perfect environment. A wrong environment or an unsuitable mix of animals in the tank, for example killer whales and penguins at the same time, will result in the death of the players pets. Instead of swimming dead at the surface or bloody death animations they simply disappear from the screen. Not playing the game for a few days and therefore not feeding, will result in a deadly surprise, too.
Using the Wi-Fi-feature of the DS the player can swap fishes with other players.